An 11-month-old boy with a functionally single ventricle, coronary sinus ostial atresia, and bilateral superior vena cava (SVC) underwent coronary sinus unroofing upon a right bidirectional cavopulmonary shunt. A persistent left SVC left open to guarantee dual coronary venous drainage became dilated with cephalocaudal blood flow reversal and desaturation. The left SVC was surgically ligated on postoperative day 35.